“Seeing” Noise: Advanced Measurement Techniques Help to Identify Sources of Turbofan Noise
Imagine sitting on one of Lake Erie’s docks on a sunny summer day when you see a jet-skier speed away. Not long afterward, waves from the wake of the jet-ski lap upon the dock.
Now, imagine you are sitting on a stator vane downstream of a turbofan rotor. As air passes through, wakes from the rotor hit the stator. Unlike the pleasant sound of Lake Erie’s waves, rotor wakes hitting turbofan stators create loud and annoying broadband and tone noise.
Rotor/stator interaction noise is the engineering term used to describe noise created this way and is one of many ways that air passing through an engine makes noise. Since we cannot see wakes in an airstream like we can see wakes in water, we must use special instrumentation like hotwire probes and Laser Doppler Velocimeters (LDV) to measure the air velocity between the rotor and the stator. Plots of the velocity measurements can be made which allow us to “see” the disturbances that the rotor created.
Since periodic disturbances results in tone noise and random turbulence generates broadband noise, detailed velocity measurements play an important role in identifying noise sources. Hotwire and LDV data are routinely used to validate calculations used to estimate noise from turbofan engines.